The present invention relates to a method for the preparation of a permanent magnet of an intermetallic compound mainly composed of a rare earth element and iron by a powder metallurgical method. More particularly, the invention relates to a method for the preparation of an alloy-type permanent magnet mainly composed of a rare earth element, denoted by a symbol R hereinbelow, iron and boron having outstandingly high stability against otherwise possible changes in the magnetic properties in the lapse of years for service.
Rare earth-based permanent magnet of the ternary alloy or intermetallic compound consisting of a rare earth element (R), iron (Fe) and boron (B) is a recently developed and very promising permanent magnet material in respect of the outstandingly high magnetic properties even as compared with the rare earth-cobalt based pe-manent magnets so that intensive investigations are now under way to develop a method for the industrial production of the permanent magnets of this type. For example, Japanese Patent Kokai Nos. 59-46008, 59-64733 and 56-89401 reported that a permanent magnet of the chemical composition of the formula Nd.sub.0.15 Fe.sub.0.77 B.sub.0.08 could have a maximum energy product (BH).sub.max of as large as 35 MGOe and a coercive force .sub.i H.sub.c of up to 10 kOe. It is taught that improvement in respect of the Curie temperature of the permanent magnet of this type can be obtained by replacing a part of the iron with cobalt Co. Further, it is taught that the coercive force .sub.i H.sub.c can be increased by the addition of a small amount of one or more of the elements selected from the group consisting of aluminum, bismuth, zirconium, hafnium, vanadium, tungsten, molybdenum, chromium, tantalum, antimony, germanium, niobium, nickel, titanium, tin and the like. Reportedly, the permanent magnets of the R-Fe-B type manufactured in a mass production system may have a maximum energy product (BH).sub.max of as large as 37 MGOe greatly exceeding the best value of 33 MGOe obtained with the rare earth-cobalt type permanent magnets. A problem in the R-Fe-B type permanent magents from the practical standpoint is that the magnets of this type are highly susceptible to oxidation in the atmospheric air so that, when the magnet is used as an element in electric or electronic instruments, the magnetic properties of the permanent magnet are gradually changed in the lapse of time to affect the performance of the instrument utilizing the permanent magnet in addition to the disadvantage that the temperature dependency of the magnets is considerably larger than in the rare earth-cobalt type permanent magnets.
In respect of the oxidation of the R-Fe-B alloys or, in particular, fine powders of such an alloy in the atmospheric air, it is a conventional practice that pulverization of the alloy ingot into powders is conducted in an atmosphere of non-oxidizing or inert gas such as nitrogen, argon and the like or in an organic solvent such as n-hexane and the like. The effectiveness of such an oxidation-preventing means is still insufficient so that oxidation of the alloy powder proceeds faster or slower throughout the processes of pulverization, transportation, storage and subsequent processing resulting in a decrease or poor reproducibility of the magnetic properties of the permanent magnets prepared from the alloy powder.
U.S. Pat. Nos. 4,597,938, 4,601,875 and 4,684,406 teach that R-Fe-B type permanent magnets with certain additive elements having improved magnetic properties can be prepared from a powder of the alloy as pulverized having an average particle size of 0.3 to 80 .mu.m. The magnetic properties reported there, however, are still not quite satisfactory. For example, the highest value of the maximum energy product is 34.5 MGOe in the magnets prepared from the alloys of the chemical composition of the formulas 63Fe5Co12B18Nd2Ta and 64Fe8Co10B16Nd2Mn. These magnets, however, are still poor in respect of the coercive force .sub.i H.sub.c with a value of 8.6 kOe or 9.3 kOe, respectively. Although some improvements can be obtained in the coercive force to give a value of about 12 kOe, the improvement is obtained only at a considerable sacrifice of the maximum energy product.